Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo sub-system to position a head over a particular track. In current disc drives, the servo fields are written onto the disc in-situ (i.e., after the disc is mounted on the spindle motor of a disc drive) when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, various types of errors prevent heads from following this ideal path. One type of error is a written-in error that arises during creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path. Written-in errors are often referred to as repeatable run-out (RRO) errors or written-in repeatable run-out (WI-RRO) errors because they cause the same errors each time the head passes along a track. In drives employing in-situ-written discs, the RRO or WI-RRO phenomenon is typically not radially-dependent, i.e., there is no definite correlation between the radial position of a track between the disc inner diameter (ID) and the disc outer diameter (OD) on the disc surface and the WI-RRO associated with the track.
To meet the demand for greater recording density in disc drives, servo-track writing is undergoing a fundamental change. In the near future, manufactured disc drives will include discs with servo-tracks that are pre-written onto the discs before the discs are mounted on the spindle motor of the drive. Tests have shown that when such discs with pre-written tracks (pre-written discs) are mounted and clamped on a spindle motor of a disc drive, in addition to WI-RRO errors, RRO errors also occur due to centering misalignment of the pre-written servo tracks and the center of rotation of the spindle, and further due to track distortion caused by disc clamping forces. This additional RRO induced in drives including pre-written discs has been found to be radially-dependent, i.e., this additional RRO varies coherently across the surface of the disc from the OD to the inner ID.
Current servo tracking systems, which are utilized with in-situ-written discs described above, are designed for tracking WI-RRO and are not suitable for tracking radially-dependent repeatable run-out (RD-RRO). Thus, when such servo systems are employed for head position control in drives with pre-written discs, the settle time required for the head before it can properly follow a destination or target track at the end of a seek operation is relatively large. This large settle time, which is due to the slow adaptation of the servo system to the RD-RRO, negatively impacts the performance of the disc drive.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.